1. Field of the Invention
The present invention relates to a vacuum carburization processing method and a vacuum carburization processing apparatus.
Priority is claimed on Japanese Patent Application No. 2006-262525, filed Sep. 27, 2006, the content of which is incorporated herein by reference.
2. Description of Related Art
Vacuum carburization process is one process of carburizing the surface layer of a metal workpiece and quenching it in order to increase its hardness. Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. Hei 8-325701) and Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2004-115893) are examples of vacuum carburization processes.
The vacuum carburization process of Patent Document 1 heats the workpiece to a predetermined temperature in a heating chamber at extremely low pressure, and carburizes the workpiece by applying a carburizing gas such as acetylene into the heating chamber. The supply of carburizing gas is stopped and the heating chamber is returned to a state of extremely low pressure, whereby carbon near the surface of the workpiece is diffused into it; after reducing the temperature to a quenching temperature, the workpiece is cooled with oil.
The vacuum carburization process of Patent Document 2 solves a problem of excessive carburization of the surface (particularly the corners) of the workpiece by supplying a decarburizing gas into a furnace (identical to the heating chamber of Patent Document 1) during initial diffusion in a vacuum carburization process such as that of Patent Document 1, thereby reducing or removing cementite on the surface of the workpiece.
In conventional vacuum carburization processes such as those mentioned above, carburization and diffusion proceed more rapidly at higher processing temperatures. Accordingly, the higher the processing temperature, the shorter the time required by the vacuum carburization process. On the other hand, when the vacuum carburization process is performed at high temperature, the crystal grains of the workpiece become enlarged. There is a problem in which the workpiece of which the crystal grains is enlarged does not have predetermined physical values.